As shown in FIG. 1, implantable medical devices (IMDs) 1, including implantable pulse generators (IPGs), implantable cardioverter-defibrillators (ICDs), implantable loop recorders (ILRs), and other subcutaneous implantable devices, use leads, e.g., insulated wires 2-4, inserted through the vasculature to the heart to provide therapy via electrodes 5-13 placed along, or at the ends of, the leads 2-4. As shown, electrodes 7, 10, and 13 are tip electrodes, electrodes 6, 9, and 12 are ring electrodes, and electrodes 5, 8, and 11 are defibrillation electrodes. Lead failures have become one of the most common modes of hardware failure in IMD systems. Statistics show that, for example, five years after implantation of a device, the failure rate for ICD leads is 15%, and that eight years after implantation, the lead failure rate increases to 40%. Causes of lead failure may include, for example, insulation breach, lead fracture, and loose set screws. Partial failure of the electrical insulation of an ICD is particularly troublesome, and may result when the lead rubs against the ICD housing, thereby thinning the insulation and potentially short-circuiting high voltage components. Meanwhile, low voltage impedance checks for detecting insulation failure can remain unaffected. Lead rubbing eventually causes lead fracture, which may have significant detrimental consequences for a cardiac patient, including inhibited pacing, inappropriate shocks, pacing capture failure, premature battery depletion, or failure to defibrillate when needed.
A conventional method of detecting lead failure entails performing a low voltage impedance check by delivering a small shock through a lead in the form of a low amplitude waveform, and then measuring the impedance. If the measured impedance is outside a prescribed “normal” range, lead failure is indicated. A second method is to check for the presence and frequency of ultra-short non-physiological sensing intervals.
The impedance measurement method of detecting lead failure has proven to be inadequate. Impedance monitoring does not reliably detect early lead malfunction, and it has been reported that impedance monitoring has failed to prevent inappropriate shocks in two thirds of patients studied. Impedance monitoring also requires specialized circuitry to source low-current pulses and measure the resulting impedance. This circuitry must also enable the IMD to deliver a therapeutic shock while substantially simultaneously making voltage and current measurements to determine the impedance. Other methods of detecting lead failure, as mentioned above, are more effective but still have difficulty in differentiating artifacts associated with lead failure from exogenous noise that may originate from other sources such as muscle activity.
Another class of IMD, for example implantable loop recorders (ILRs), does not use leads to sense cardiac activity. Instead, ILRs detect far-field cardiac signals present in a subcutaneous ECG (SECG) signal. (The term ECG as used hereinafter may refer to electrocardiograms, subcutaneous electrocardiograms, or intracardiac electrograms.) Thus, in addition to detecting lead failure, achieving optimum performance of an IMD requires the ability to recognize noise in an SECG or in other lead signals. In particular, it is desirable to recognize a central feature of interest within an ECG signal, known to those skilled in the art as a “QRS complex.” Near-field noise is capable of corrupting ECG signals in IMDs, and may cause over- or under-detection of cardiac arrhythmias. Although detection of noise is critical to device performance, existing solutions to account for noise in an implantable loop recorder are inadequate. To prevent noise, implantable cardiac devices may be placed at a location where the heart signal is maximized, but this imposes an additional burden on the physician. It is also known that noise may be reduced through the use of standard filtering techniques or by manually adjusting sensing parameters. However, existing devices sense the ECG using only a single pair of electrodes, i.e., using a single signal vector or channel. Brignole et al., Journal of Cardiovascular Electrophysiology, 19: 928-934, 2008, describe a sensing method for use in an ILR in which sensing thresholds for ECG signals are adapted to the peak of the R-wave within the QRS complex, and the threshold decreases to a minimum value. Although this method is new for ILRs, it is common practice in other devices such as IPGs and ICDs. Furthermore, although both high frequencies and amplifier saturation are sensed to further reduce false signal detection, relying on a single channel and only adapting it to the peak of the R-wave are insufficient. Even if noise is correctly detected, the ILR is susceptible to remaining in noise mode if the minimum threshold is set too low. Conversely, the ILR is susceptible to under-sensing if the minimum threshold is set too high. Often, the minimum threshold is fixed by the physician at the time of implantation of the device and cannot be changed. Methods that rely on these fixed QRS sensing thresholds, or even methods such as that presented in U.S. Patent Application Publication No. 2009/0187227A1 (which discloses the use of dynamic detection thresholds), can incur significant risk depending on the basis for changing those thresholds. Furthermore, arrhythmia classifications that are based on QRS detection are at increased risk for being incorrect if noise is mistaken for a true signal. Finally, another drawback of these methods is that they do not sense general signal quality, nor do they detect whether or not the signal to noise ratio is high enough to reliably differentiate noise from the signal itself.
What is needed is a method of reducing noise-induced false QRS signal detection, along with an improved method of detecting faulty leads.